1. Field of the Invention
The present invention relates, generally speaking, to the analysis and correction of wave surfaces in real time and, more particularly, to an interferometer with shearing or lateral duplication that serves in measuring the phase distortion of a wave-front.
Wave surfaces analysis includes the measurement of the phase differences in the wave delivered by an optical system or instrument with respect to a reference wave surface that would have resulted from the same optical system if assumed to be perfect and unaffected by the atmospheric turbulence.
The wave can be adversely affected by aberrations in the instrument (e.g. in the case of a very large mirror becoming defomed under the effect of various stresses, or a mosaric system made up of multiple mirrors) or by phenomena related to atmospheric propagation (turbulence, thermal defocalization).
The advantage of wavefront analysis in real time is to be able to apply a correction instantaneously to the wave surface using a deformable mirror or, more generally, an adaptive optical system, and thus to free the system of the aforesaid disturbences.
2. Description of the Prior Art
In the current state of the art, wavefront analysis and correction systems working in real time have two types of application both requiring very high spatial resolution (i.e., less than or equal to 100 .mu.rad), and fine aiming for a laser beam and very long range imagery (astronomy, satellite observation). In these applicational fields, the apertures in questions are approximately one meter in dimension and the adaptive optical arrangement makes it possible to reach a resolution limit dependent solely on diffraction and no longer on phase defects.
In high-power laser aiming systems based on what are called "return waves", use is made of a wave surface analyzer on the basis of which a deformable mirror is positioned in order to transmit a conjugate wave of the wave received and which focuses perfectly on the target.
As far as very long range optical imagery systems are concerned, it is possible with the wave surface analyzing device to detect phase distorsions in the wave front falling on the entrance pupil into the optical system, and a deformable mirror is controlled by the analyzing device in order to correct the wave front for the distorsions thus detected.
Optical systems are known, as shown by the U.S. Pat. No. 3,923,400 to Hardy, for example, for forming the image of an object through the atmosphere, where such systems comprise a device working in real time for detecting and correcting the phase of the wavefront as seen by these optical systems, said device comprising:
(a) interferometer means based on shearing or lateral duplication for receiving the disturbed wavefront, determining the relative phase differences in real time between the various regions in thus disturbed wavefront and producing signals representing the said phase differences;
(b) means responding to the said phase-difference signals for simultaneously producing, in parallel and in real time, signals representing phase corrections to be made to the various regions in the disturbed wavefront in order to obtain a corrected wavefront; and
(c) phase correcting means receiving the disturbed wavefront and responding to the phase correcting signals to modify the phase differences in real time between the various regins of the disturbed wave front and change the shape of this disturbed wave front to generate a corrected wave front.
The purpose of the lateral-duplication interferometer is to duplicate the wavefront to measure the phase difference between the two overlapping regions of the wavefront. To measure this phase difference, at the output from a detector placed in the interference plane, an interference signal is created with modulation thereof obtained by moving one of the portions of the lateral-duplication interferometer.
In the aforesaid Hardy patent, the lateral-shear interferometer is formed of an optical diffraction grating producing two cones in two slightly divergent angular directions with a common area of overlap. The wavefront is therefore duplicated and it is possible to measure the phase difference between two neighbouring areas of the wavefront. This is achieved by modulating the interference pattern and by detecting the relative phase of this interference pattern at various points therein. The use in the aforesaid Hardy patent of an interferometer with lateral shear and diffraction grating results in difficulties in tuning the interferometer due to the existence of different diffraction orders within a grating and the need for overlap between just two orders, only 0 and 1 or -1 and 0.
The diffraction gratings in the prior art are moved either in translation or in rotation (cf. "Radial Grating Shear Heterodyne Interferometer", by Chris L. Koliopoulos, Apllied Optics, May 1, 1980, vol. 19, no. 9, pages 1523 and sq.) for modulating the interference pattern. The interference in higher orders gives rise to frequency modulations that are multiples of the basic modulation frequency that must be filtered.
At a point in the interference plane, the interference is observed after filtering between the points M(x) and M(x+.DELTA.x) and between M(x) and M(x-.DELTA.x). An exact calculation shows that the phases of these two modulations are identical if the phase shifts .phi.(x)-.phi.(x+.DELTA.x) and .phi.(x-.DELTA.x)-.phi.(x) are identical, i.e. if the wave is locally planar, which is something of a hindrance.